Conventionally, a sheath material and a partially insulating material for electric wires often make use of polyvinyl chloride (PVC), and its flexibility, flame retardancy and insulating properties have been appraised. Generally, PVC contains a large amount of plasticizer so that when the plasticizer is lost by heating etc., the PVC is easily hardened and generates a chlorine-based gas upon combustion, and thus development for electric wires which can be substituted for PVC has been desired in recent years.
Under these circumstances, various flame-retardant resin compositions based on ethylene-based polymers such as polyethylene have been proposed.
U.S. Pat. No. 6,232,377 describes a flame-retardant resin composition which comprises a specific ethylene-based copolymer selected from an ethylene/vinyl ester copolymer, an ethylene/α,β-unsaturated carboxylic acid copolymer and low-density polyethylene, and further comprises a metal hydroxide, a triazine ring containing compound and a specific flame-retardant compound. However, these ethylene-based polymers have a problem that pliability and flexibility are easily lowered when the amount of inorganic compounds such as metal oxides is increased in order to increase the flam retardant effect.
Accordingly, a first object of the present invention is to provide a resin composition excellent in pliability and flexibility and superior in tensile physical properties, a molded product comprising the same, particularly an insulating material and/or sheath for electric wires.
On one hand, many kinds of thermoplastic polymers and thermosetting polymers have been used in internal wires in home appliances, buildings, interior decorations, automobile parts, and electronic instruments. A majority of these polymers (particularly olefin polymers) are easily flammable.
From the viewpoint of protection against disasters, there is increasing demand for incombustibility and flame retardancy of various facilities and structures, and particularly high flame retardancy is required of home appliances that can be the origin of a fire. Criteria for flame retardancy of internal wire materials are stipulated by for example UL standards in the US (Underwriters' Laboratories Inc.) etc., and evaluated by a vertical flame test called VW-1 test. Accordingly, materials usable for a long time even exposure to high temperatures and fires are desired, and a method of conferring high flame retardancy on many thermoplastic polymers and thermosetting polymers by adding a flame retardant in production of the polymers or in production of molded products is widely used.
As the flame retardant, many compounds such as metal hydroxides, borates, organic halogenated compounds, phosphorus compounds such as phosphates, red phosphorus, organic phosphorus compounds etc. and organic nitrogen compounds are used. Among these, organic halogenated compounds, organic phosphorus compounds etc. exhibit an excellent flame retardant effect.
However, these halogen-containing compounds have a problem that they are pyrolyzed at the time of molding resin to generate hydrogen halide, to deteriorate the resin itself thus causing coloration, or to generate hydrogen halide on the occasion of a fire.
As a halogen-free flame retardant, an inorganic flame retardant such as aluminum hydroxide, magnesium hydroxide or the like is conventionally used. When only the inorganic compound is used, however, the flame retardant effect is low, and a large amount of the flame retardant is required to exhibit a sufficient effect, but when it is added in a large amount, physical properties inherent in resin may be deteriorated, and thus its application is limited.
As halogen-free flame-retardants exhibiting a relatively excellent flame retardant effect, there are specific organic phosphorus compounds and specific organic nitrogen compounds, and these are also often practically used.
The conventional organic phosphate flame retardant is represented by triphenyl phosphate (referred to hereinafter as “TPP”), but this compound is poor in heat resistance and highly volatile, and is thus not suitable for resin to be molded at high temperatures, and particularly because of pollution of a molding die, its application is limited.
As compounds used as flame retardants hardly volatilizing organic phosphorus, there are condensed phosphates described in JP-B 51-19858, JP-A 59-202240 etc. These compounds are superior to TPP in respect of heat resistance and low volatilization, but do not surpass TPP in flame retardant effect per unit weight of phosphorus, and therefore there is a problem that these should thus be added in a large amount, and thus the temperature of thermal deformation is significantly lowered due to the effect of the plasticizer for resin.
A large number of compositions using flame-retardants based on condensed polyphosphates such as ammonium polyphosphate and polyphosphoric amides are also proposed (JP-A54-22450, JP-A 9-316250 etc.). However, polyphosphoric acid absorbs water to reduce electrical resistance gradually due to water absorption and is thus not suitable as an insulating covering material for electric wire/cable etc., and therefore its application is limited.
To prevent nutrient enrichment in closed water systems in lakes and marshes, compositions substituted for phosphorus flame-retardants are also required in recent years.
Organic nitrogen compounds such as melamine also exhibit a relatively high flame retardant effect (JP-A 8-176343 etc.). However, these compounds have been often used in combination with the phosphorus flame retardant in order to achieve a higher flame retardant effect.
Accordingly, a second object of the present invention is to provide a polymer composition having high flame retardancy without containing a halogen- or phosphorus-based flame retardant, particularly a flame retardant polymer composition suitable as a covering material or sheath for electric wires.